Agreed, once it is put together it would be more like a finished product, but as long as you make the Arduino-compatible MPU re-programmable (break out a USB mini connector w/ an ftdi IC) you would be ok. To make it slightly modular you could add a break-out port (somehow) that allows for either a bluetooth module, x-bee module, or wifi module as you already have a unified breakout board for all 3 of them. Would let users get hand data to pretty much any wireless enabled device of their choosing and no hardware modifications would need to be made.
Lateral Finger Motion: I’m not a sign language expert, but a quick google search of the sign language alphabet shows that it might however I don’t think that would be a hard problem to fix. You can’t bend the flex sensors sideways but you can add conductive fabric on the sides of the fingers that could act as switches to indicate whether two fingers are touching and de-bounce them in software.
Different sizes: You can go with the un-spoken average glove size, which is simply the size of gloves that random companies hand out at running races in goodie bags. Boulder Running Company and Runners Roost seem to always be giving out gloves and they seem to fit most people. If you avoid putting sensors on the finger tips you can also make finger-less gloves. The flex sensor doesn’t need to run along the entire length of the finger to get proper readings.